It is well known that a voltage regulator diode referred to as a Zener diode is usable for generating a standard voltage or for voltage stabilization. The Zener diode utilizes the property of a pn junction which, when subjected to an inverse voltage of no less than a predetermined value, undergoes Zener breakdown (generally below 5 V) or avalanche breakdown (generally not less than 5 V) to drastically decrease in its internal resistance, so that voltage fluctuates only little with increasing current.
A typical prior art Zener diode has such a structure as shown in FIG. 4 of the accompanying drawings. The Zener diode designated by reference numeral 100 in FIG. 4 comprises a substrate which includes a highly doped N-type impurity diffusion layer 101 (N+ layer), and an N-type impurity diffusion layer 102 epitaxially formed on the N+ layer 101. A P-type impurity diffusion region 103 is annularly formed in a surface portion of the N-type layer 102 of the substrate, and a highly doped P-type region 104 (P+ region) is formed at the center of the annular P-type region 103. The surface of the substrate is covered by an insulating layer made of silicon dioxide, and at a portion of the insulating layer corresponding to the the P+ region is formed an opening where an aluminum electrode 105 is formed. The insulating layer 106 is covered by a passivation film 107.
With the above-described structure, when a voltage Vz (breakdown voltage) beyond a predetermined value is applied between the N+ layer 101 (actually an unillustrated electrode held in contact with the N+ layer 101) and the aluminum electrode 105, a breakdown takes place at the pn junction to abruptly allow current passage. The voltage Vz in this condition fluctuates only little with increasing current and is therefore usable for voltage stabilization and etc.
However, the prior art Zener diode having the above structure still has the following problems.
First, with the prior art Zener diode, since the width of the depletion layer varies depending on the dopant concentrations on both sides of the pn junction, it is necessary to adjust the rated breakdown voltage Vz by adjusting the respective dopant concentrations in the N-type layer 102 and the P+ region 104 in the course of the manufacturing process. However, since the dopant concentration in the P+ region 104 is extremely high (not less than about 10.sup.20 /cm.sup.3), an attempt to accurately adjust the concentration by implanting a counted number of dopants will result in an extreme cost increase. Thus, it is extremely difficult to accurately adjust the rated breakdown voltage Vz in the course of the manufacturing process.
Secondly, the rated breakdown voltage Vz of the prior art Zener diode is to be set at a low value of about 2-3 V, the P+ region 104 must be made to have a super high dopant concentration of about 10.sup.21 /cm.sup.3. Such a super high concentration is likely to cause defects in the crystalline substrate, consequently leading a problem of a large leak current in addition to difficulty of concentration adjustment.
In the third place, with the prior art Zener diode, the N-type layer 102 having a relatively low dopant concentration is interposed between the N+ layer 101 and the P+ region 104, and a current flows in the N-type layer 102 when the rated breakdown voltage Vz is applied. Though the internal resistance decreases abruptly upon breakdown of the pn junction, the N-type layer 102 having a relatively low dopant density still exhibits a non-negligible resistance. Therefore, the intended voltage retention cannot be sharply realized due to the resistance of the N-type layer 102 which causes a voltage increase attendant with a current increase.